The present invention relates to the geophysical methods employed to estimate parameters of the subsoil, notably in the context of hydrocarbon exploration and production.
It relates more particularly to the so-called 4D seismic techniques. In these techniques, there are first seismic recordings, obtained in a first phase during a campaign of “base” measurements (“base survey”), for example before a hydrocarbon reservoir is placed in production, and there is a subsequent campaign of measurements (“monitor survey”), for example after a few years of operation of the reservoir, to obtain second seismic recordings. The base and monitor seismic recordings (or seismic traces) are compared to estimate variations of physical parameters of the geological layers in the area explored.
The parameters of which the variations are thus estimated may comprise the density ρ, the speed VP of propagation of the pressure waves (P waves) and the speed VS of propagation of the shear waves (S waves) in the media forming the various geological layers of the area explored. Reference is often made to the seismic impedances in each medium, IP=ρ×VP and IS=ρ×VS, which govern the propagation of the P waves and S waves in the layers. Because of the changes linked to oil production, for example the replacement of oil with water or gas, the parameters ρ, VP, VS are modified in certain layers. This results in changes of amplitude in the seismic recordings, and in time offsets in the recorded seismic traces. The comparative analysis of the recordings comprises an inversion to estimate the variations of the parameters in order to obtain an idea of the levels of saturation in the layers being operated.
An inversion method that can be used to analyze the time offsets in the base and monitor seismic traces (dependant on the variations of propagation speeds) at the same time as the changes of amplitude (dependant on the impedance variations) is described in EP 1 865 340 A1.
A 3D seismic technique, i.e. a technique based on a single survey, taking into account well logs is described in the U.S. Pat. No. 5,798,982, which also mentions an extension of the technique to 4D seismic by a comparison of inverted seismic blocks.
Another 4D seismic data analysis method, described in WO 2008/140655 A1, uses a model-based inversion in one or more wells where logs have been recorded. The document does not describe the inversion method or the way in which the model is parameterized. The results of the inversion are then extended by moving away from the well, using a statistical method. A correlation computation is performed in order to relate the time reference of the monitor recordings to that of the base recordings. With the model used, the method seeks to directly estimate variations of saturation levels and pressure variations in the geological layers.
An object of the invention is to add to the 4D seismic techniques, notably by making them take into account geological and dynamic constraints.